1. Field of the Invention
The present invention relates to a heterodyne receiver, and a communication system using the heterodyne receiver.
2. Description of Related Art
Heterodyne receivers having a downconverter for downconverting a received radio signal (may be referred to as high frequency signal hereinafter) to an IF (Intermediate Frequency) signal before demodulating this received high-frequency signal are widely used.
FIG. 5 shows a structure of a common heterodyne receiver. This heterodyne receiver includes an antenna 101, a bandpass filter 102, a low noise amplifier 103, a local oscillator 104, a mixer 105, a bandpass filter 106, and a demodulator 107.
A radio signal received by the antenna 101 is band-limited by the bandpass filter 102, and then amplified by the amplifier 103. The signal amplified by the amplifier 103 is mixed with (or multiplied by) a local oscillation signal having a fixed frequency outputted from the local oscillator 104 to be downconverted into an IF signal. The IF signal is band-limited by the bandpass filter 106, and then demodulated by the demodulator 107 to produce demodulated data.
When the frequency of a radio signal intended to be received and demodulated is F1, and the frequency of the local oscillation signal generated by the local oscillator 104 is F2, the frequency F3 of the downconverted IF signal becomes F1−F2. Accordingly, in a case where F1 is 315 MHz and F3 is set at 10.7 MHz, it is necessary for the local oscillator 104 to generate an oscillation signal having a frequency of F2=F1−F3=304.3 MHz. The local oscillator 104 can generate the 304.3 MHz oscillation signal through about 5 times frequency multiplication on a 60 MHz oscillation signal obtained by use of quartz, for example.
It is also known to constitute such a local oscillator by a PLL circuit as described, for example, in Japanese Patent Application Laid-open No. 2000-286738.
Incidentally, vehicles are becoming equipped with different in-vehicle wireless systems such as the tire pressure monitoring system, remote key entry system and smart key system. Accordingly, it is becoming necessary to mount different receives corresponding to these different systems on each vehicle, because these systems use different frequencies.